Theme Showcase: Fix copy on upsell #6545879

Changes proposed in this Pull Request

• Update wording on Theme Showcase upsell to reflect the correct upgrade path for uploading themes (Business and eCommerce, not Premium and Business)

Before

After

Testing instructions

• Switch to this PR on a free simple site
• Go to /themes/[site]
• You should see the upsell underneath the search area

Fixes #55988